Low modulus composite materials have been employed as femoral components of hip implants to reduce stress shielding of the bone and consequently reduce bone tissue resorption. Currently, composite implants are stabilized in their bony bed by a press fit. With this method of stabilization, however, optimum stress distribution effects are not fully realized.
Several attempts have been made to improve the fixation of composite femoral implants to bone. These include porous polymer coatings and particulate bioactive coatings. Implants using porous polymer coatings seek to achieve fixation through mechanical interlocking between the implant and surrounding bone tissue, while the bioactive coatings are designed to attain fixation through a chemical bond between the implant and bone.
Implant surfaces coated with polysulfone particles in an effort to create a porous coating which would behave similarly to a porous metal coating are disclosed in M. Spector, et al., "Porous Polymers for Biological Fixation," Clin. Ortho. Rel. Res., 235:207-218 (1988). Although it is disclosed that some bone growth was evident, the majority of the tissue about the implant surface was fibrous. The porous polymer did not enhance the bone tissue growth in any way.
Composite system of calcium phosphate ceramic powder pressed onto a polymer surface and then cured are also known. See P. Boone, et al., "Bone Attachment of HA Coated Polymers," J. Biomed. Mater. Res. 23, No. A2:183-199 (1989); and M. Zimmerman, et al., "The Attachment of Hydroxyapatite Coated Polysulfone to Bone," J. Appl. Biomat., 1:295-305 (1990). These systems are provided in two fashions. First, the ceramic is flush with the polymer surface, hence, only bonding occurs. Second, the calcium phosphate particles extend from the polymer surface. When interfacial bonding is tested, the failure is between the polymer and the calcium phosphate particles. Hence, the interface between the calcium phosphate particles and the polymer is the weak link in the system. These references disclose the use of polyurethane thermoset and polysulfone thermoplastic polymers, a number of other polymers are similarly used as a matrix for a filler of calcium phosphate ceramic powder in U.S. Pat. No. 4,202,055--Reiner et al. The ceramic particles at the surface of this implant resorb and are replaced by bone tissue. There are no structural fibers and the polymer alone is intended to bear the load. This limits the load-bearing applications of this material to those of the polymer. An implantable bone fixation device comprised of an absorbable polymer and a calcium phosphate ceramic powder filler material is disclosed in U.S. Pat. No. 4,781,183--Casey et al. The device disclosed is a temporary load bearing device which resorbs upon implantation. The calcium phosphate particles are added for strength and also resorb, therefore this device is not fixed to bone tissue through the chemical bonding of bioactive material or porous ingrowth.
Structural fibers will improve certain mechanical properties of composite materials. For example, U.S. Pat. No. 4,239,113--Gross et al. discloses a composition of methylmethacrylate polymers and a bioactive ceramic powder combined with vitreous mineral fibers less than 20 millimeters long. This device is used as a grouting material to bond implants to bone tissue. The chopped fibers are not specifically tailored or designed for mechanical property optimization. A similar composition is disclosed in U.S. Pat. No. 4,131,597--Bluethgen et al., which mentions the use of glass or carbon fibers to add strength to the composite. This patent, however, does not specifically discuss placing fibers to achieve bone bonding regionally. Also, no method of optimization of material properties through arrangement of the structural fibers is suggested. Finally, the method of fixation to be achieved by the disclosed material is not explained.
A similar approach using a textured device of carbon fiber/triazin, coated or non-coated with calcium phosphate particles is discussed in G. Maistrelli, et al., "Hydroxyapatite Coating on Carbon Composite Hip Implants in Dogs," J. Bone Jt. Surg., 74-B:452-456 (1992). The results reported show a higher degree of bone contact for the coated devices after six months. However, longer studies are needed to evaluate the long term fatigue effects on the triazin/calcium phosphate interface.
In all these prior art systems, however, it has been found that although a bond between the substrate polymer and bone may be achieved through the use of a bioactive material at the interface, the resulting implant is still unsatisfactory. As discussed above, the significant limitation remains the interfacial bond between the bioactive material and the polymer.
Much of the prior art discussed immediately above utilized calcium phosphate ceramic powders as the bioactive component of the composite. Bioactive glass materials were developed by Hench in 1969. See L. Hench, et al., "Bonding Mechanisms at the Interface of Ceramic Prosthetic Materials," J. Biomed. Mater. Res., 2:117-141 (1971). More recently, elongated, continuous bioactive glass fibers have been fabricated. See U. Pazzaglia, et al., "Study of the Osteoconductive Properties of Bioactive Glass Fibers," J. Biomed. Mater. Res., 23:1289-1297 (1989); and H. Tagai, et al., "Preparation of Apatite Glass Fiber for Application as Biomaterials," Ceramics in Surgery, Vincenzini, P. (Ed.), Amsterdam, Elsevier Sci. Pub. Co. (1983), p. 387-393. The latter reference discloses bioactive glass fibers in resorbable bone plates.
As seen from the foregoing, it would be desirable to provide a composite material for use as a prosthetic device that could be designed to provide a structural modulus that closely matched bone. It is thus an object of the present invention to provide composite structures that incorporate a bioactive material in a polymer matrix along with a structural fiber to provide adequate strength. Additionally, it is a further object of the present invention to provide three dimensional and hybrid composite materials that overcome the deficiencies of the prior art, and in particular that provide an adequate interfacial bond between the bioactive material and the polymer.